For the studies of the expression of milk proteins during the functional development of the rat mammary gland and in mammary tumor MTW9, the cDNA sequences complementary to milk protein mRNAs were cloned in E. coli cells and the clones characterized. The DNA sequence of alpha-LA cDNA clone analysis shows that rat alpha-LA has 17 extra residues beyond the C-terminus of the alpha-lactalbumin isolated and sequenced to date from other species. The predicted C-terminal sequence is hydrophobic, proline rich and bears some resemblance to beta-casein sequences. Additional results show that 1) more than one plasmid DNA with differences in the restriction maps have been identified for several of the milk proteins except for alpha-LA, suggesting either a presence of a family of genes or allelic differences for these proteins; 2) the expression of individual milk proteins is dependent on the functional stage of the gland; 3) there is an inverse relationship between the expression of milk proteins and the methylation of the gene sequences; 4) mammotrophic hormones required for synthesis and stability of milk proteins and their mRNAs, when withdrawn arrest the synthesis alpha-LA in the mammary tumor MTW9 at 6 h or earlier of withdrawal but without any measurable effect on other proteins of the tumor.